Lars Hofhansl created HBASE-9840:
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Summary: Large scans and BlockCache evictions problems
Key: HBASE-9840
URL: https://issues.apache.org/jira/browse/HBASE-9840
Project: HBase
Issue Type: Bug
Reporter: Lars Hofhansl
I just ran into a scenario that baffled me first, but after some reflection
makes sense. I ran a very large scan that filled up most of the block cache
with my scan's data. I ran that scan a few times.
That I ran a smaller scan, and this scan will never get all its blocks cached
if it does not fit into the remaining BlockCache, regardless how I often I run
it.
The reason is that the block of the first large scan were all promoted. Since
the 2nd scan did not fully fit into the cache all block a round-robin evicted
as I rerun the scan, and those blocks will never get accessed more than once
before they get evicted again.
Since promoted blocks are not demoted the large scan's block will never be
evicted unless we have another small enough scan that can promote its blocks.
Not sure what the proper solution is, but it seems only a LRU cache that can
expire blocks over time would solve.
Granted, this is a pretty special case.
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